1. Field of the Invention
This invention relates to a tap holder comprising a main portion having a shank which is to be coupled with a main shaft of a machine tool, a quick change portion secured to said main portion, and a tool holding portion which is detachably coupled with said quick change portion and has a torque control function. Such a tap holder is mainly used for a machining center.
2. Description of the Related Art
A tap holder comprising a main portion having a shank which is to be secured to an axial hole of a main shaft of a machine tool, a tool holding portion having a torque control function and a quick change portion which couples said main portion and tool holding portion with each other has been disclosed in Japanese Patent Publication Nos. 50-5,102 and 58-41,974. In such a known tap holder, upon exchanging a tool, the quick change mechanism is moved toward the main shaft of the machine tool, so that when a tapping torque exceeds a nominal value and the clutch is intermittently brought into the racing, there is a danger that the tool holding portion might be removed from the quick change portion.
In the tap chuck described in said Japanese Patent Publication No. 50-5102, the torque control mechanism of the tool holding portion is constituted by a clutch including a torque adjusting cam and a torque adjusting member made of hard polyurethane rubber, and the torque control is effected by urging a movable cam against a fixed cam by means of a resilient compression force of the hard polyurethane rubber. During the operation, when the clutch is brought into the racing, the fixed cam is inserted and removed intermittently into and out of a recess, so that the hard polyurethane rubber is brought into the compressed and non-compressed conditions alternately, which affects the normal operation of the quick change portion. That is to say, the tool holding portion is coupled with a float shaft of the quick change portion by means of steel balls and the coupling can be released by moving a clamp collar toward the main shaft of the machine tool so that the steel balls can move freely in a radial direction of the float shaft. However, in the racing condition, since the intermittent force generated by the torque control mechanism functions to pull the tool holding portion toward the main portion, the intermittent force serves to rotate the steel balls and move the clamp collar toward the main portion, the tool holding portion is decoupled from the quick change portion and the tool holding portion might be left on a workpiece.
In the tool holder disclosed in said Japanese Patent Publication No. 58-41,974, the torque setting is effected with the aid of a belleville spring. In this known tool holder, since use is made of the same quick change mechanism as that explained above, the above mentioned drawback could not be overcome.
In the tap holder used in the machining center, the tool holding portion, i.e. a tap collet must comprise a means for preventing the overload from being transferred to the tap or tool. Such an overload preventing means has been described in Japanese Utility Model Laid-open Publication Nos. 55-142,222 and 59-32,724. However, these known tap holders have such drawbacks that they could not be used for a long time due to a, rapid, wear of friction parts and sufficient safety could not be attained.